Avian influenza
[2][3] Symptoms of avian influenza vary according to both the strain of virus underlying the infection, and on the species of bird or mammal affected.
[4] Chickens infected with LPAI display mild symptoms or are asymptomatic, whereas HPAI causes serious breathing difficulties, significant drop in egg production, and sudden death.
[11] Classification of an avian virus strain as HPAI or LPAI does not predict how serious the disease might be if it infects humans or other mammals.
[11][13] Since 2006, the World Organization for Animal Health requires all LPAI H5 and H7 detections to be reported because of their potential to mutate into highly pathogenic strains.
[19][20] There is evidence that the virus can survive for long periods in freshwater after being excreted in feces by its avian host, and can withstand prolonged freezing.
[24] Further variations exist within the subtypes and can lead to very significant differences in the virus's ability to infect and cause disease.
Symptoms vary from mild to severe (including death), but as of December 2024 there have been no observed instances of sustained human-human transmission.
[4][17][30] There are a number of factors that generally prevent avian influenza viruses from causing epidemics in humans or other mammals.
[31][32][33] Influenza viruses are constantly changing as small genetic mutations accumulate, a process known as antigenic drift.
Over time, mutation may lead to a change in antigenic properties such that host antibodies (acquired through vaccination or prior infection) do not provide effective protection, causing a fresh outbreak of disease.
[11] Between 1959 and 1995, there were 15 recorded outbreaks of highly pathogenic avian influenza (HPAI) in poultry, with losses varying from a few birds on a single farm to many millions.
[11] Since then, several virus strains (both LPAI and HPAI) have become endemic among wild birds with increasingly frequent outbreaks among domestic poultry, especially of the H5 and H7 subtypes.
Birds – Influenza A viruses of various subtypes have a large reservoir in wild waterbirds of the orders Anseriformes (for example, ducks, geese, and swans) and Charadriiformes (for example, gulls, terns, and waders) which can infect the respiratory and gastrointestinal tract without affecting the health of the host.
[49] All subtypes of avian Influenza A have potential to cross the species barrier, with H5N1 and H7N9 considered the biggest threats.
People working with birds, such as conservationists or poultry workers, are advised to wear appropriate personal protection equipment.
[61] As of June 2024, there is concern about two subtypes of avian influenza which are circulating in wild bird populations worldwide, H5N1 and H7N9.
Both of these have potential to devastate poultry stocks, and both have jumped to humans with relatively high case fatality rates.
[81] It was discovered that a low pathogenic strain of A/H7N9 was circulating among chickens, and that all the affected people had been exposed in poultry markets.
[83][84] In order to contain the HPAI outbreak, the Chinese authorities in 2017 initiated a large scale vaccination campaign against avian influenza in poultry.
It has demonstrated antigenic drift to evade vaccines, and remains a potential threat to the poultry industry and public health.
The strain in these outbreaks was of North American origin and is unrelated to the Asian lineage H7N9 which is associated with human infections in China.
[88] During April 2024, avian influenza was first detected in dairy cows in several US states and subsequently spread more widely through the year.
[89][90][91] It was shown that the virus can persist on milking equipment, which provides a probable transmission route for cow-to-cow and cow-to-human spread.
[93] According to CDC, 7% of 115 dairy workers had evidence of recent infection in a study from Michigan and Colorado from June to August 2024 – half of them asymptomatic.
New networks of laboratories have emerged to detect and respond to avian flu, such as the Crisis Management Center for Animal Health, the Global Avian Influenza Network for Surveillance, OFFLU, and the Global Early Warning System for major animal diseases.
[101] Cooperative measures created in response to HPAI have served as a basis for programs related to other emerging and re-emerging infectious diseases.
[102] In Vietnam, policymakers, with the support of the Food and Agriculture Organization of the United Nations (FAO), used HPAI control to accelerate the industrialization of livestock production for export by proposing to increase the portion of large-scale commercial farms and reducing the number of poultry keepers from 8 to 2 million by 2010.
Backyard production appeared to hold greater risk than commercial production due to lack of biosecurity and close contact with humans, though HPAI spread in intensively raised flocks was greater due to high density rearing and genetic homogeneity.
[104] A report by FAO totalled economic losses caused by avian influenza in South East Asia up to 2005 around US$10 billion.
[106] As poultry serves as a source of food security and liquid assets, the most vulnerable populations were poor, small scale farmers.
